Nowadays, due to catalyst improvements and electronic mixture control of last generation vehicles equipped with internal combustion engine, the most significant part of the total emissions of carbon monoxide and unburned hydrocarbons takes place during the cold phase, if compared with those exhausted in hot conditions, with a clear consequence on air quality of urban contexts. The purpose of this research, developed by the Department of Industrial Engineering of the University of Naples Federico II with reference to an European background, is a deeper analysis of the engine and after-treatment system behaviour within the cold start transient and the evaluation of cold start additional emissions: a methodology was developed and optimized to evaluate the cold transient duration, the emitted quantities during the cold phase and the relevant time-dependence function.The whole procedure was applied on the exhaust emissions of one scooter belonging to the Euro-3 legislative category, equipped with catalytic converter, and with a displacement of 280 cm3. Experimental tests were performed on a chassis dynamometer in cold and hot functioning conditions, during both driving cycle regulated by law and real world driving cycles. The study was focused on this specific vehicle because the pertinent information aren't very extensive in scientific literature, with particular reference to the appraisal of real world and cold-start extra emissions of this particular two-wheeler vehicular class (Euro-3 legislative category and displacement between 250 and 750 cm3), which in the last years is taking on an increasing chief role in private mobility.